A conventional continuous vacuum deposition apparatus comprises an evaporation chamber in which a metal to be deposited, such as zinc, for instance, is vaporized and a channel which leads the vapor of the metal to the surface of a substrate metal sheet, such as a steel strip for instance, which runs at the opening of the channel and a shutter means is provided at the junction of said evaporation chamber and said vapor channel. The shutter is either of a butterfly type or of a sliding type.
Control of the deposition amount (coating weight) is effected by changing the electric power supplied to the heater of the melting bath and conditioning the aperture of the shutter means. With such means, however, the change in the deposition amount is gradual and takes some time; it cannot be instantaneously changed. This means that the transitional portions of the metal sheet have a coating of varying thickness, and must therefore be scrapped, which is a waste of material.
This invention is intended to solve the above-described problem of the prior art and to provide a method for instantaneously controlling the metal vapor supply in a vacuum deposition apparatus.
We have found that the above problem can be solved by utilizing the so-called choke phenomenon. That is, the vapor flow can be instantaneously changed by choking the flow (providing the flow with acoustic velocity), thus enabling the deposition amount to be changed rapidly.